The Severe Acute Respiratory Syndrome (SARS) causes significant mortality and no effective vaccine is available. Viruses closely related to SARS coronavirus (SARS-CoV) have been found in bats in several parts of the world, making re-emergence of SARS a distinct possibility. We propose to develop a safe inactivated and second-generation vaccine to prevent SARS. We showed that virus deleted in the small envelope (E) protein (rSARS-CoV-Delta E), or in this E protein plus six additional genes (6, 7a, 7b, 8a, 8b, and 9b) accessory for replication were attenuated in hamsters and mice. Among the deleted genes, E gene was the main one responsible for virus attenuation in the animal models tested. Thus, virus with either deleted or mutated E protein serves as a useful platform for the production of a chemically inactivated vaccine, and for the engineering of a live attenuated anti-SARS vaccine. We have shown that SARS-CoV E protein regulates host stress and unfolded protein responses and, consequent immune responses to the virus. We propose the identification and construction of rSARS-CoV including E protein mutants that elicit strong immune responses and are still attenuated. The specific aims of this project are: (i) The construction of rSARS-CoV viruses with modified E protein mutants eliciting higher immune responses to the virus by maintaining E protein's role in morphogenesis. We hypothesize that preventing the interaction of E protein with host cell proteins involved in stress response signaling will lead to an enhanced immune response. E protein mutant construction will be based on the generation of an rSARS-CoV with a mutator phenotype. (ii) To test rSARSCoV-E* as vaccine candidates in mice using a mouse adapted prototype. We will analyze the immune responses to selected vaccine candidates, and the influence of non-essential group specific genes and of age. (iii) To increase the safety and titer of selected rSARS-CoV-E* vaccine candidates by modification of replicase genes, and to increase vaccine candidate titers using a virus with mutator phenotype.